Small Optical Package Having Multiple Optically Aligned Soldered Elements Therein

ABSTRACT

A thermally stable small optical package is disclosed housing optically aligned elements forming an optical component. The elements are all directly soldered to a base member or directly soldered to a supporting member soldered to a common base member. The sealed container has a supporting base member of stainless steel and a first optical fiber mount is directly soldered to the supporting base. A first optical fiber is directly soldered to the upper end of the first optical fiber mount. A similar arrangement is provided wherein a second optical fiber mount is directly soldered to the base and a second optical fiber is directly soldered to the upper end of the second optical fiber mount. A frequency doubling crystal is directly soldered to the stainless steel base member after being aligned with the two optical fibers. This design is inexpensive to manufacture and provides a thermally stable component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent ApplicationNo. 60/869,972 filed Dec. 14, 2006, entitled “A packaging design forfiber coupled nonlinear optical waveguide device” which is incorporatedherein by reference for all purposes.

FIELD OF THE INVENTION

This invention relates to a substantially thermally stable small opticalpackage that houses a plurality of optically aligned elements forming anoptical component, wherein the elements are all directly soldered to abase member or directly soldered to a supporting member soldered to acommon base member.

Optical devices and components operational within optical systems aretypically subjected to various thermal and mechanical loads and orstresses during their lifetime. An example of such optical devices is anoptical filter assembly. An assembly of this type comprises two opticalglass fibers inserted into capillary ferrules to produce fiber-ferrulesub-assemblies aligned with an optical filter assembly. The opticalcomponents of the filter assembly generally have graded-index (GRIN)lenses embedded into an insulating glass tube, which in turn aremechanically protected by a metal housing. These filter assemblies oftenexhibit insertion losses higher than desired, resulting in degradedoverall performance of the communications system or module. The problemis particularly acute during exposure to ambient operating conditionswhere temperature is variable.

Input glass ferrules generally employ one of two designs. A singlecapillary suitable for containing multiple glass fibers or separatecircular capillaries for each fiber have been used, each with relativelyshort fiber-receiving conical lead-in ends. With such input ferrules,the optical fiber is subjected to an unwanted S-bending over the shortconical end. This excessive micro bending increases the insertionlosses. Fiber-ferrule subassemblies employing such ferrules aremanufactured by inserting the optical fibers stripped of their polymercoating into the respective ferrule capillaries; epoxy bonding thefibers into the ferrule capillaries, including the conical end portions;grinding and polishing an angled facet on the fiber-ferrule; anddepositing on the polished surface an anti-reflection (AR) coating. Oncefinished, the fiber-ferrule is aligned and assembled with thecollimating GRIN lens and then embedded into the insulating glass tube,which, in turn, is protected by a metal housing.

In many instances adhesives are used to secure fiber-ferrules to a baseor support member within their package. In some instances thesefiber-ferrules are laser welded to a support member.

A precise alignment achieved during initial assembly of a filter priorto final packaging can be easily decreased due to the adhesive curingprocess and the high temperature thermal cycles associated with laserwelding during the final packaging of the components. Such manufacturingprocesses and resulting components have several problems resulting fromstresses on the optical components due to the thermal contraction as aresult of a thermal mismatch between the glass and metal materials,polymerization shrinkage in adhesive bonds, and structural constraintsinduced by bonding during encapsulation. These stresses lead todisplacements of optical components during bonding, resulting in 0.3 to1 dB or greater increases in the insertion loss.

In an attempt to provide a packaged component that is thermally stableand that will perform to required specifications over the lifetime ofthe device, adhesives such as epoxies, or bonding by laser welding wasnot considered practicable. Each of these has their associateddrawbacks. Although Laser welding requires little set-up time andsetting time, post weld shift, a known problem, can cause fibers housedin sleeves welded to support members to shift.

FIG. 1 is a diagram of an optical package that is the result of years ofdevelopment in an attempt to automate the assembly process and to lessenmanufacturing costs while providing a relatively small, thermally stableoptically packaged component. The goal was to provide a packagedcomponent wherein performance would not significantly degrade over theexpected lifetime of the device. Problems related to cost, andreliability have resulted in the abandonment of this solution.

In FIG. 1 a non-linear frequency doubling crystal 10 performs filteringof an input signal and outputs a frequency doubled optical signal. Inthe manufacture of the component, optical fibers 12 a and 12 b held insleeves must be precisely aligned so that light launched from the inputfiber 12 a to its output and through the frequency doubling crystal 10must couple without undue loss into the output receiving optical fiber12 b and provide optimum filtering through the crystal 10.

In FIG. 1 a frequency doubling optical component is shown, having aninput optical fiber 12 a held in an optical sleeve or ferrule 14 a,optically coupled to a frequency doubling crystal 10, and opticallycoupled to an output optical fiber 12 b. The optical sleeve 14 a is helddown in a fixed position by clamp 16 which is laser welded to a basemember beneath. The optical crystal is secured to a holder 18 which isclamped to the base by a clamp 7. Bolts 9 secure sub-base members to thepackage. It is evident that the numerous interfaces between elementswithin the package increase the likelihood of misalignment ofcomponents. Furthermore, having this many elements and fasteners withinthis package lessens available space for other components. The packageshown in FIG. 1 is conveniently shown before the top is positioned andhermetically seals the package.

It is apparent from looking at the device in FIG. 1, that there arenumerous interfaces wherein components are fixedly coupled to othercomponents, and wherein these other components must be relativelyaligned and secured to a common base member. Furthermore, the use ofadhesives, laser welding, soldering, and clamping were all required inthe assembly of this device.

Understandably, the more components that hold other components within anoptical package, the greater the probability for misalignment during thelifetime of the device.

A simple scenario follows:

If an optical fiber is housed within a sleeve and the sleeve is bondedto a base member, there is a possibility that there will be relativemovement of the optical fiber and the sleeve. Furthermore, there is thepossibility that there will be relative movement between the opticalsleeve and the base member.

However if the optical fiber is directly bonded to the base member,there is only the possibility of relative movement between the opticalfiber and the base member. In this regard, it is preferable lessen thenumber of interfaces.

When optical components made of crystalline material are to be fixedlysecured to a base, secure bonding is essential, especially if acrystalline slab is to be optically aligned with an input and an outputoptical waveguide or optical fiber to maximize output power and toachieve a desired filtering such as frequency doubling. Furthermore,since this crystalline material is birefringent, it is important thatthe material does not undergo a change in stress as a result oftemperature variation due to a thermal mismatch between the base and thecrystalline material.

Therefore, it is an object of this invention, to provide a packagehousing components wherein the components are optically aligned withincertain tolerances, and wherein the components remain optically alignedwithin those tolerances.

It is also an object of the invention to provide a package with fewerinterfaces than in the prior art device shown in FIG. 1, and to providea package which due to having fewer elements and interfaces, providesspace to place a cooler/heater within the package to maintain thetemperature of the components within predetermined tolerances.

This result of this improvement over the prior art, is a package whichhas a better cooling response time, and which is relatively inexpensiveto manufacture while providing accurate alignment throughout the workinglife of the device.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention, there is provided, asealed container, comprising;

-   -   a) a supporting base member consisting of a first material;    -   b) a first optical fiber mount having a height h₁, wherein the        first optical fiber mount has a lower end and an upper end;    -   c) a first optical fiber directly soldered to the upper end of        the first optical fiber mount;    -   d) a second optical fiber mount having a lower end and an upper        end and having a height h₂;    -   e) a second optical fiber directly soldered to the upper end of        the second optical fiber mount; and,    -   f) a thin optical element for modifying light passing        therethrough mounted in the container, wherein the thin optical        element has a planar bottom surface, and wherein the planar        bottom surface is directly soldered to the base member, wherein        only solder is present between the planar bottom surface and the        base member, and, wherein the first and second optical fibers        are aligned with the thin wafer so that light launched into one        of the first and second optical fibers from an end extending        from the sealed container passes through the thin optical        element and exits the other of the first and second optical        fibers extending out of the sealed container.

In accordance with another aspect of the invention, there is provided, amethod of manufacturing a sealed container, comprising:

-   -   a) providing a supporting base member made of a first material;    -   b) providing a first optical fiber mount having a height h₁,        wherein the first optical fiber mount has a lower end and an        upper end;    -   c) providing a first optical fiber;    -   d) directly soldering the first optical fiber to the upper end        of the first optical fiber mount;    -   e) providing a second optical fiber mount having a lower end and        an upper end and having a height h₂;    -   f) providing a second optical fiber;    -   g) directly soldering the second optical fiber to the upper end        of the second optical fiber mount; and,    -   h) providing a thin optical element having a planar bottom        surface, for modifying light passing therethrough; and,    -   i) mounting the thin optical element in the container by        directly soldering the planar bottom surface and the base member        together, wherein the first and second optical fibers are        aligned with the thin wafer so that light launched into one of        the first and second optical fibers from an end extending from        the sealed container passes through the thin optical element and        exits the other of the first and second optical fibers extending        out of the sealed container.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described inconjunction with the drawings in which:

FIG. 1 is a view illustrating a conventional packaged component having aplurality of interfaces.

FIG. 2 is a view of a packaged component in accordance with anembodiment of this invention wherein fewer parts and interfaces arerequired and wherein a filter is soldered to the package base.

FIG. 3 is a schematic side view of a base member having an input lensedoptical fiber optically aligned with an output optical lensed fiberhaving a PPLN (poled lithium niobate) crystal there between wherein theoptical fibers and the crystal are soldered to their supporting members.

FIG. 4 is a top view of the components shown in FIG. 3.

DETAILED DESCRIPTION

Turning now to FIG. 2 a photograph of a non-sealed container inaccordance with the invention is shown, prior to placing andhermetically sealing a lid in place over the device. The device of FIG.2 provides the same functionality as the device of FIG. 1. However thedevice of FIG. 1 requires the thermal electric cooler to be outside ofthe package due to the limited space. In FIG. 2 an input optical fiber22 a is shown soldered in place on a base 21 precisely aligned with thenon-linear frequency doubling crystal 20 and with the output opticalfiber 22 b. The solder ball made of AuSn (gold tin) 24 a is shownresting upon a low pedestal 23 a is preferably a ZrO2 ceramic such asytterbium stabilized zirconium, which is also soldered to the base 21 ofthe container. Only solder is present between the optical fiber 22 a andan upper end of the pedestal 23 a, and only solder is present betweenthe lower end of the pedestal 23 a and the base 21 of the container. Thefrequency doubling crystal 20 is also directly soldered to the base 21upon which it rests, so that the planar lower surface of the crystal 20securely mates with the container having only solder there between. Thissolder is applied by using a reflow technique. A similar arrangement isshown at the output end wherein optical fiber 22 b is directly solderedto its respective pedestal 23 b, which is directly soldered to the base21 of the package. No ferrules are required to house and hold theoptical fibers 22 a and 22 b, lessening the possibility of misalignment.A thermal electric cooler (TEC) 25 is disposed beneath the base 21 andwithin the package, so that efficient cooling and a more rapid coolingresponse time can be obtained. Electrodes 29 are provided to power theTEC 25. This particular device requires the temperature to be maintainedat a predetermined temperature that is within ±0.01 C.

Providing lenses on the end of the optical fibers 22 a and 22 b alsolessen the component count within the package and reduces the couplinglosses that may be associated with gluing GRIN lenses to fiber ends oradjusting optical fiber end relatively with GRIN lenses that are notrequired in this device. A thermistor 26 is also disposed within thepackage on the base 21 for temperature monitoring.

The base member 21, preferably including or consisting of stainlesssteel, preferably KT-stainless steel SST 403, has a coefficient ofthermal expansion (CTE) that is substantially matched to the CTE of thecrystal 20 to less than 5 ppm/K. If the CTE of the crystal 20 was notsuitably matched to the CTE of the base member, the crystal 20 would beadversely stressed upon cooling down from the 160° C. solderingtemperature. BiSn solder was used in securing the crystal 20 to thebase.

FIGS. 3 and 4 illustrate the optical input and output optical fibers 32a, 32 b, the pedestals or optical fiber mounts 33 a and 33 b, and theperiodically poled lithium niobate frequency doubling crystal 30 in moredetail.

In FIG. 3 the optical fiber 32 a has an output end adjacent to thecrystal 30 shown to have a pointed or chiseled lens tip to focus andcouple light from the fiber 32 a to a region near the surface of thecrystal 30. Furthermore, optical fiber 32 a is securely mounted to thepedestal 33 a via a single solder ball 34 a, made of a solder with arelatively high melting temperature T_(h)295 C. The fiber mount 33 a issecured to the base member 31 by way of having a very thin layer ofsolder between the mount 33 a and the base member 31. A solder reflowtechnique is used to fixedly attach the base member 31 with the fibermount 33 a. The frequency doubling crystal 30 is also soldered to thebase member 31; however, with a lower melting temperature solder, e.g.BiSn solder, having a melting temperature of approximately 160 C wellbelow Th. Thus, the crystal 30 can be relatively moved and alignedduring manufacture without the risk of moving the fiber mounts 32 a and32 b, which are first secured to the base 31 with high meltingtemperature solder of about 295 C, e.g. AuSn solder.

This much more simple and elegant design having only solder joints tosecure elements directly, without the use of tubes, sleeves, holders orclamps, provides a more robust device which is easier and less costly tomanufacture.

The base member 31, which preferably consists of stainless steel, has acoefficient of thermal expansion (CTE) that is substantially matched tothe CTE of the crystal 30 to less than 5 ppm/K. The CTE differencebetween periodically poled lithium niobate (PPLN) and SST403 is smallerthan 2 ppm/k. If the CTE of the crystal 30 was not suitably matched tothe CTE of the base member, the crystal 20 would be adversely stressedupon cooling down from the 160° C. soldering temperature.

Various alignment techniques can be used to manufacture this device,such as moving one optical fiber, e.g. 32 a, while keeping the otheroptical fiber, e.g. 32 b, and the crystal 30 fixed by first securingthem with solder to the base 31. Preferably this is done while applyinga signal to the device via one of the optical fibers and monitoringpower at the output end. Once an optimum alignment is achieved theoutput fiber capturing the test light can be soldered in place.

The process for aligning the device in FIGS. 2 and 3 is performed by thefollowing steps:

-   -   1. Attaching the two fiber mounts (23 a and 23 b) and thermistor        26 to the base element 21 using AuSn solder having a melting        temperature of 295 C using a hot plate.    -   2. Attaching the base element (after step 1) to the top plate of        TEC (25) and attaching the bottom plate of TEC to the bottom        surface of the package using indium (160 C) in a reflow oven;        these two attachments are done in one step.    -   3. Attaching PPLN crystal waveguide sliver (20) to the top        surface of the base element using BiSn (160 C) on hot plate.    -   4. Wire bonding the TEC and thermistor to the package    -   5. Aligning the input fiber (22 a) and soldering it on the fiber        mount (23 a) with AuSn (24 a).    -   6. Aligning the output fiber (22 b) and soldering it on the        fiber mount (23 b) with AuSn (24 b).

Advantageously, this invention provides a method and device wherein twooptical fibers are aligned with an optical element where all three arefixed to a common base using solder. The numerous advantages of aredescribed heretofore.

Of course other embodiments can be envisaged without departing from thespirit and scope of this invention.

1. A sealed container, comprising: a) a supporting base member made of afirst material; b) a first optical fiber mount having a height h₁,wherein the first optical fiber mount has a lower end and an upper end;c) a first optical fiber directly soldered to the upper end of the firstoptical fiber mount; d) a second optical fiber mount having a lower endand an upper end and having a height h₂; e) a second optical fiberdirectly soldered to the upper end of the second optical fiber mount;and, f) a thin optical element for modifying light passing therethroughmounted in the container, wherein the thin optical element has a planarbottom surface, and wherein the planar bottom surface is directlysoldered to the base member, wherein only solder is present between theplanar bottom surface and the base member, and, wherein the first andsecond optical fibers are aligned with the thin wafer so that lightlaunched into one of the first and second optical fibers from an endextending from the sealed container passes through the thin opticalelement and exits the other of the first and second optical fibersextending out of the sealed container.
 2. A sealed container as definedin claim 1, wherein the lower end of the first optical fiber mount isdirectly soldered to the base member so that only solder is presentbetween the lower end of the first fiber mount and the base member; and,wherein the lower end of the second optical fiber mount is directlysoldered to the base member so that only solder is present between thelower end of the second fiber mount and the base member.
 3. A sealedcontainer as defined in claim 2, wherein h₁, is approximately equal toh₂.
 4. A sealed container as defined in claim 2, wherein the basesupport member is fixedly coupled to the bottom of the sealed container,and wherein the sealed container is hermetically sealed.
 5. A sealedcontainer as defined in claim 2 wherein the CTE of the first material ismatched to the CTE of the thin optical element to within 3-5 ppm/K.
 6. Asealed container as defined in claim 5 wherein the thin optical elementcomprises a crystalline material.
 7. A sealed container as defined inclaim 5 wherein the thin optical element comprises a birefringentmaterial.
 8. A sealed container as defined in claim 2 wherein the firstand second fiber mounts are soldered to the base member with a firstsolder; and the thin wafer is soldered to the base member with a secondsolder, different from the first solder.
 9. A sealed container asdefined in claim 8 wherein second solder has a lower melting point thanthe first solder to enable the first and second fiber mounts to beadjusted during assembly, while keeping the thin wafer fixed to the base10. A sealed container as defined in claim 9, wherein the second soldercomprises BiSn solder; and wherein the first solder comprises AuSnsolder.
 11. A sealed container as defined in claim 2 further comprisinga thermal electric cooler disposed within the sealed container forcooling and maintaining the temperature within the sealed container towithin predetermined limits.
 12. A sealed container as defined in claim2 wherein the first material comprises stainless steel.
 13. A method ofmanufacturing a sealed container, comprising: a) providing a supportingbase member made of a first material; b) providing a first optical fibermount having a height h₁, wherein the first optical fiber mount has alower end and an upper end; c) providing a first optical fiber; d)directly soldering the first optical fiber to the upper end of the firstoptical fiber mount; d) providing a second optical fiber mount having alower end and an upper end and having a height h₂; e) providing a secondoptical fiber; f) directly soldering the second optical fiber to theupper end of the second optical fiber mount; and, g) providing a thinoptical element having a planar bottom surface, for modifying lightpassing therethrough; and, h) mounting the thin optical element in thecontainer by directly soldering the planar bottom surface and the basemember together, wherein the first and second optical fibers are alignedwith the thin wafer so that light launched into one of the first andsecond optical fibers from an end extending from the sealed containerpasses through the thin optical element and exits the other of the firstand second optical fibers extending out of the sealed container.
 14. Amethod as defined in claim 13, wherein the solder used for soldering thethin optical element with the base member, has a lower meltingtemperature than the solder used to solder the first optical fiber withthe first optical fiber mount.